fix: use openjsf.org Slack invite subdomain (#6500)

* fix: use openjsf.org Slack invite subdomain

* chore: consistently use branch name in navigation GitHub links

* chore: use dedicated security policy link in footer

Author: MattIPv4

navigation.json

@@ -39,7 +39,7 @@
       "text": "components.containers.footer.links.codeOfConduct"
     },
     {
-      "link": "https://github.com/nodejs/node/blob/HEAD/SECURITY.md#security",
+      "link": "https://github.com/nodejs/node/security/policy",
       "text": "components.containers.footer.links.security"
     },
     {
@@ -65,7 +65,7 @@
     },
     {
       "icon": "slack",
-      "link": "https://openjs-foundation.slack.com/join/shared_invite/zt-238w9sb83-Qk9NcsrEMomq94Y~3gW8EQ#/shared-invite/email",
+      "link": "https://slack-invite.openjsf.org/",
       "alt": "Slack"
     },
     {
@@ -120,7 +120,7 @@
           "label": "components.navigation.getInvolved.links.contribute"
         },
         "codeOfConduct": {
-          "link": "https://github.com/nodejs/admin/blob/HEAD/CODE_OF_CONDUCT.md",
+          "link": "https://github.com/nodejs/admin/blob/main/CODE_OF_CONDUCT.md",
           "label": "components.navigation.getInvolved.links.codeOfConduct"
         }
       }